This invention relates to an exhaust gas purification catalyst, more particularly to usage of hollow oxide powder for a catalytic layer.
As the catalyst for purifying exhaust gas from an internal combustion engine, it is common to use precious metals such as Pt, Pd or Rh which is loaded on oxide powder such as alumina that is referred to as support material. Alternatively ceria may be adopted as the support material as well considering its oxygen storage capacity. Further, catalytic layers having the support materials may consist of two layers as shown in U.S. Pat. No. 5,019,546 or Japanese Patent Application Publication no. S63-258648.
It is also known to use hollow oxide powder as the support material in order to enhance catalyst performance as shown in Japanese Patent Application Publication nos. 2000-203830 or 2001-347167. By using the hollow oxide powder as support material, porous volume of the catalytic layer is increased thereby enhancing gas diffusion to improve exhaust gas purification performance at high temperature.
Further, Japanese Patent Application Publication no. 2002-001120 shows that both of the hollow and solid oxide powders can be used as the support material. In the case of multi layer structure of the support material, for enhancement of the gas diffusion, it is preferred to arrange the hollow oxide powder in a layer except for the lowermost layer among the multiple layers and it is particularly preferred to arrange it in the uppermost layer.
Such a hollow oxide powder is formed by gathering together or agglutinating super fine crystals of a size of several to tens of nanometers to make spaces of its shell wall where exhaust gas may pass through. However, there is a certain level of resistance against the exhaust gas flow. Accordingly, when exhaust gas pressure is low, e.g. in an engine idle condition just after engine start, the improvement of exhaust gas purification performance by the gas diffusion can not be obtained enough. Especially, in a cold engine start condition, the hollow oxide powder contained in the upper layer of catalyst functions as a heat insulator, thereby heat transfer from the exhaust gas to the lower layer is deteriorated, and, also since heat is deprived further from the lower layer to the honeycomb-shaped carrier, temperature rise of the lower layer is delayed which is a disadvantage in terms of activation of catalyst in cold start condition (light-off performance).
Therefore, the inventors herein have recognized the above-mentioned disadvantages and have developed a method to improve the catalyst containing the hollow oxide powder.